Memory cards, which can be used for non-volatile storage, may be applied to the acquisition and processing of sounds/images in digital format (Digital Audio/Video). The use of these cards is of great interest in that they form easily transportable media of limited overall dimensions and with considerable storage capacity. A further significant advantage of memory cards over other storage media, such as magnetic tapes, is that they do not require mechanical parts that move, which are subject to wear and rapid consumption.
These memory cards can be inserted directly into a system for acquiring sounds/images in digital format, such as a DSC (Digital Still Camera), a video camera with DSC functionality, or an audio recorder for music and speech which transfers the sounds/images, after they have been acquired and digitized, to the memory card.
In general, with respect to sounds/images in digital format, the data stored in the memory card is subsequently transferred by a host processor apparatus, such as an ordinary personal computer, to media having a higher capacity, for example, a hard disk of the personal computer. In the host processor apparatus, the data can be processed by programs which require a computing power and a storage capacity which cannot be provided in the acquisition apparatus mentioned above. In such cases, memory cards are used for the temporary storage of the acquired data.
To make the memory card usable for the subsequent storage of new data, it has to be subjected to an operation of erasing the data previously stored on it. Typically, this erasure does not take place in the acquisition system itself, but is carried out in the host processor apparatus used for transferring the data from the card to the larger-capacity memory (off-line erasure). A memory card of the type currently used for sounds/images in digital format comprises an outer casing incorporating an integrated circuit made from semiconductor material comprising one or more semiconductor memories, suitable for non-volatile data storage.
The memory cards currently in use conform to physical specifications set by international standards or by standards established by the manufacturers themselves, which make them compatible with personal computers. One of these standards has been established by the PCMCIA (Personal Computer Memory Card International Association).
The integrated circuits used in conventional memory cards use non-volatile semiconductor memories which are electrically erasable and programmable, such as the EEPROM type (Electrically Erasable Programmable Read Only Memory), or, preferably, the Flash EEPPROM type. Recently, memory cards using read-only non-volatile semiconductor memories (ROM) have also become available, with the data written permanently onto them during the manufacture of the integrated circuit (Mask ROM). Memory cards of the latter type are used, for example, as storage media for music, in the same way as music CDs.
The erasure of the data stored in electrically erasable and programmable memories, and, in particular, in Flash memories with internal architecture of the type known as NOR, requires, as is known, an appropriate pre-programming operation that includes a preliminary programming of all the memory cells in such a way as to bring them to the same logic level in order to prevent any of the memory cells from entering a state of depletion during the erasure.
After this pre-programming, the electrical erasure of the stored data can take place. The control of the operations of erasing and programming Flash memories requires the presence of a microprocessor provided with a corresponding microprogram which may be of considerable complexity. Flash memories with architecture of the type known as NAND do not require pre-programming of the memory cells to be subjected to erasure. However, memories with this second type of architecture have certain drawbacks with respect to the former type, especially with respect to the speed of reading the stored data.
An example of a memory card including Flash memories is described in U.S. Pat. No. 5,663,901. In this patent, with reference to FIG. 13A, a description is given of a memory card using a plurality of memories of the Flash EEPROM type. These memories are associated with a controller module provided with a microprocessor for controlling the exchange of data between the EEPROM memories and the external apparatus, which act as hosts for the memory card via interface storage registers. The controller module also comprises a controller for the memories, which is provided, in turn, with a timing signal generator.
It should be noted that the necessity of using microprocessors for the implementation of particular memory control procedures, as well as the intrinsic complexity of Flash memories, make the production cost of memory cards particularly high. Furthermore, the use of complex controllers makes it very difficult to integrate the controllers on a single semiconductor chip, particularly on the chip used for the Flash memories, and requires the formation of electrical interconnections for the transfer of the data between the various chips, thus further increasing the costs of design and production.
This is reflected in the final cost of the systems for acquiring and processing sounds/images which make use of memory cards, thus limiting their distribution essentially to professional use.
It should also be noted that the electrical erasure and programming functionality offered by Flash memories according to the procedures dictated by the technological characteristics of the memory does not appear to be imposed by requirements emerging from their application to the field of the acquisition of sounds/images in digital format. In this field of application, complete erasure of the memory before a subsequent use is required in all cases.